THE CURRENT CONTEXT OF LEAN AND SIX SIGMA LOGISTICS APPLICATIONS IN LITERATURE : A SYSTEMATIC REVIEW

Originated in the Toyota Production System, the concept of lean production has become widely disseminated and broadly applied in the industry. When considering lean and six sigma principles in logistics, companies have obtained advantages in terms of cost and quality, which encouraged scientific researches for improvements in the topic. To study the current state of the art on Lean Logistics (LL) and Six Sigma Logistics (SSL), in the national and international level, a systematic review of lean and six sigma logistics were performed, aiming to evaluate theoretical and practical applications, in order to classify the current literature and propose studies to address research opportunities. For this purpose, a review protocol was created to guide and structure the review. As results, it was identified that Transportation has been the main logistic operational drive component in which LL has being applied. The most frequent type of waste considered was Motion and most of the studies were practical applications. Finally, this study found considerable research opportunities for future studies.


INTRODUCTION
Based on the Toyota Production System (TPS), Lean Manufacturing had its origin after World War II, when Toyota, in order to outstand from its competitors in terms of quality, reinvented the mass production model, focusing on eliminating time and material waste in its production process.Consequently, Toyota could obtain fast and flexible processes, allowing it to offer customers what they want, when they want it, with high quality and at a reasonable price (Liker, 2005).
In the economic context, companies have been searching for the development of continuous improvement habits, waste elimination and improvement of the skills in their human resources, and those practices were related to Lean thinking (Gonçales Filho et al., 2014).The concept of Lean has been widely applied in production process, with a considerable number of studies in this field (Dennis, 2008).However, there have been difficulties in adapting Lean concepts and assumptions to Logistics, although lean philosophy can potentially provide benefits for this field (Zylstra, 2008).Therefore, there are opportunities for the development of studies that map and generate information on the current literature state in the Lean and Six Sigma Logistics area, such as systematic reviews.
In this sense, the review in Gonçales Filho et al. (2014) comprehended the strategy in Lean manufacturing in different industry segments.As a result, it was observed the Processing Industry was the most representative, followed by the Automobile and Service Industries.Furthermore, the authors identified opportunities for studies on specific Industries such as Sugarcane-based Energy, which represented a high potential of application.
Similarly, Khorasani et al. (2015) performed a systematic review on Lean Supply Chain Management applied to the Healthcare context.The papers were categorized according to their relevance, scope, methodology, type of industry and year of publication, and results demonstrated that the selected papers focused on the application of Lean methodology to reduce costs, increase time efficiency and quality.In addition, the most studied topics were identified as being technology in the Supply Chain and its implementation factors, and the main methodologies were data mining, surveys, simulation, optimization models and Value Stream Mapping.The authors suggest future studies on practical application of theoretical articles.
In this context, considering the research opportunities for connecting Lean, Six Sigma and Logistic concepts in the literature, this study aimed to characterize the current state of the literature on Lean Logistics (LL) and Six Sigma Logistics (SSL), using a systematic review approach for analyzing pa-pers published from 2012 to 2016.For this purpose, along with the review protocol, research questions were provided to guide data collection from the selected papers in terms of theory, application, Lean tools, Six Sigma concepts, methodology and Logistic concepts.Therefore, the contribution of this study comprehends the identification of the main topics covered in literature of LL and SSL as well as new research opportunities for the practices.
This study is organized as follows: Section 1 provides a brief introduction to the study, justifying its contribution and objectives; Section 2 presents a detailed description of the systematic review methodology; Section 3 contains the analysis of data collected from the selected papers and the discussion on the LL and SSL current context.Finally, in Section 4, conclusions and final thoughts on the results achieved are exposed.

METHODOLOGY
It is important to follow proper guidelines to conduct a systematic review to ensure a valid and unbiased analysis.Therefore, a systematic review protocol was elaborated with guidelines for the entire review process.Afterwards, following the inclusion and exclusion criteria, papers were selected to compose the final analysis.Finally, a complete reading and analysis on those papers was performed to collect information to answer a set of research questions and describe the current panorama of LL e SSL studies.These methodology steps are systematically illustrated in Figure 1.
The research protocol that led this systematic review is summarized in Table 1.In order to clarify the criteria adopted in the protocol, it is important to comment the connection between lean and six sigma considered in the review.
The combination of lean and six sigma concepts results in a powerful combination of tools, techniques and methods for management and control analysis.According to Goldsby et Martichenko (2005), combining lean concepts and techniques with six sigma methods from logistics perspective results in waste reduction caused by regulated efforts to understand and decrease variation, simultaneously increasing velocity and material flow performance in the logistics and supply chain.
The papers resulted from the search using keywords and the Boolean operators were matched with the protocol guidelines, and those aligned with the inclusion criteria were selected for this review.It is worth mentioning the necessity of a subjective analysis in this phase: the sensibility and knowledge of the authors about the subject, since some articles did not directly state the approach of LL or SSL, making necessary to analyze carefully the published studies' content.Papers were selected with full accordance between the authors, and duplicates were removed.

Articles on Lean only
Source: The author(s)' own The next step was to perform a detailed reading on the selected papers to extract their respective information based on a set of research questions (RQ), as shown in Table 2, along with their respective objectives.These RQ aimed to identify trends and gaps in the current literature.
For RQ-1, it was proposed a set of multiple-choice alternatives: a) Logistics and Lean or six sigma concepts are linked (when both concepts are connected, presenting lean or six sigma logistics integrated theories).b) The concepts are not linked (when it presents either only concepts of logistics, or lean thinking, or six sigma or both, but not relating them clearly in the literature review).c) It does not contain specific literature review topics (when none of the concepts is presented in a specific theoretical topic or the literature review section could not be identified in the paper).
In RQ-2, the answer could be one or more logistics operational drivers, since it is possible to approach more than one simultaneously.Therefore, the answer options were: a) Transportation; b) Inventory; c) Facility; d) Information; and e) Not defined, in cases where there was not a clear definition or the study was not restricted to these drivers.RQ-3 allowed the classification of the selected papers according its applicability.The answer was "applied" when the paper's results were composed of purely practical content application in some sector or organization -although the sector was not clearly defined in some cases.On the other hand, when results were composed of only theoretical content, without practical application, the paper was considered theoretical.
For RQ-4, it could be identified more than one tool/principle in each paper.For the cases when there was no application of a certain tool or it was not explicitly specified, the question was not applicable.The range of tools was adapted from Negrão et al. (2016).
RQ-5 intended to gather the paper's application sectors within the articles.The range of economic and industrial activity sector was defined according to the National Economic Activity Code (CNAE 2.0) of the Brazilian Institute of Geography and Statistics (IBGE, 2017).If a sector was not clearly specified, then the answer for this was "not defined".If the paper was theoretical and not applied in any sector, the answer was "not applicable".

Research Question Objective
RQ-1 In the paper's literature review, was there connection between logistics and lean thinking concepts?
Verify the level of integration between the logistics and lean thinking concepts

RQ-2
Which logistics operational driver is the lean logistics principle applied in?
Identify which logistics operational drivers (facility, inventory, transportation and information) the lean philosophy has been applied more frequently.

RQ-3
Is the paper a theoretical research or an application in some productive/ industrial sector?
Define what is more common: theoretical studies on lean logistics application and impacts, or effective establishment of lean philosophy in the proposition of specific logistics results

RQ-4
What lean logistics tools and/or principles does the article addresses?
Identify how logistics has been applied in organizations based on the lean tools utilized and if there is a trend on its utilization

RQ-5
What productive/industrial/economic sector the article focuses on?
Identify in which productive, industrial or economy sectors the lean logistics approach has been more commonly applied.

RQ-6
What wastes tackled by lean logistics does the article address?
Define the main problems and wastes that motivate proposal of studies and application of lean logistics Source: The author(s)' own Finally, in RQ-6, the alternatives for answer were based on the seven wastes defined by Goldsby et Martichenko (2005), which are: Inventory; Transportation; Space and Facilities; Time; Packaging; Administration; and Knowledge.When it was not clear what waste was being tackled, the answer would be "not defined".It was possible to select more than one alternative, since it is common to tackle more than one type of waste simultaneously in lean systems.

RESULTS AND ANALYSIS
The results of the systematic review were presented in this section, which comprises the information obtained from collection and analysis of the selected papers based on the proposed protocol and research questions, aiming to verify the current scenario of the LL and SSL literature.A set of parameters was created for analysis (research question's answers) for each research question (RQ), thus, for the systematic review.In Figure 2 all sets of criteria and their respective code are described.
Based on the review protocol, it was obtained a sample of 67 papers, varying in terms of language, country of publication and lean or six sigma logistics approach.In Table 3, a summary about those papers is presented, along with each RQ analysis.
In terms of language, one can verify the majority of papers were written in English, representing 64%, followed by 28% in Portuguese, and 8% in Spanish, as illustrated in

RQ-6 RQ-5 RQ-4 RQ-3 RQ-2 RQ-1 Country Authors
Development of a tool for quantifying value-added shares in material supply as a main task of logistics.

Filho et Lobosco, 2015
There is a lack of studies regarding in-plant milk run system as a whole, and in certain areas of the system, such as manual feeding.
There is also a tendency to deviate from lean manufacturing philosophy, looking for optimality based on restrictive objectives All respondents in a survey reported varying degrees of cost savings and productivity improvements with Lean and Six Sigma.Large logistics companies are more likely to implement Lean and SS, and the ones with a higher service standard are more likely to On this matter, this analysis considered only those concepts presented in a specific section in the articles that should be dedicated for literature review.As shown in Figure 4, most of the papers did not connect the concepts.Only 24 out of 67 effectively connected them, creating an important foundation for the lean or six sigma logistics literature.On the other hand, in 10 papers it could not be identified specific literature review sections or topics.
In general, there is little information on how the concepts relate, influence and assist each other.This is evidence of a weak theoretical basis about the combination between the ideas, which indicates lack of integration among the concepts that originated Lean or Six Sigma Logistics.

RQ-2: Which logistics operational driver was the lean or six sigma logistics principle applied in?
The papers were categorized in three different groups, regarding the presence of the logistics operational drives approach in them: 1) The paper refers to one single driver (29 papers); 2) There was a combination of two or more drivers (23 papers) and; 3) No driver was identified in the paper (15 papers).This classification is shown in Figure 5.The papers, in which any focus on logistics drivers was not identified, represented 22%, which is an expressive amount of papers, considering the importance of logistics drivers in many aspects.Among the papers in the second category, there was a high variation in terms of combinations of logistics drivers.The most frequent one was "Transportation and Inventory", found in 5 papers, followed by "Transportation, Inventory, Information, and Facility" and "Transportation and Facility", found in 4 papers each.It is notable that Transportation is the logistics operational driver present in more combinations, stating its importance by itself, and its impact and influence in a more holistic view is considered.
The classification of papers per logistics driver is shown in Figure 6.Transportation was the most frequent logistics driver addressed in lean or six sigma logistics, present in 32 papers, both individually and combined with others.This represents almost 50% of the sample of papers analyzed.Following Transportation, Inventory was the second driver most addressed.When it comes to the representativeness of the other logistics components, one can observe that Facility is present in 18 papers; and Information comprehended 18 papers.
The aggregation of the quantity of papers in each driver presents a large number of papers, when compared to the total considered in this review, since the combinations repeated for each driver are considered in the paper.Example, article "A" was counted in Information totals' only; meanwhile article "B" was counted in both Transportation and Inventory totals.
Therefore, this analysis could describe the representativeness and approach of each logistics operational drivers in the LL or SSL context.Transportation was presented as the main logistics driver, according to the current panorama of the literature.However, it is important to emphasize that combining driver allows a more systemic and integrated view of the problem to be solved, often being the most appropriate approach, even though it is more complex.Thus, the idea of more research on LL and SSL with combined approach of logistics drivers becomes interesting.The answers in RQ-3 (Figure 7) demonstrated that most of the published research is applied: 51 out of 67 papers (76%) in the sample were essentially applied research, focusing on a specific productive or industrial sector, although, in some cases, that sector was not clearly stated.On the other hand, 16 papers (24%) were purely theoretical research.Many of the papers applied to some sector did not have theoretical studies and were fundamentally composed of practical approaches.There is a lack of studies and publications on LL or SSL theory.It is valid to consider that theoretical studies are an important basis for the applied studies.The fact that most publications currently focus on the application of these concepts and philosophies demonstrates a strong trend to the practical perspective of these studies.14, Número 4, 2017, pp. 586-602 DOI: 10.14488/BJOPM.2017.v14.n4.a14 598 Kanban was the most used tool, presented in 15 papers, followed by Six Sigma and Milk-Run in 14 papers each, JIT in 12 papers, VSM (Value Stream Mapping) in 11 papers, 5S in 10 papers, and so on.The predominance of Kanban among the other tools is due to its high importance in the lean thinking philosophy, which clearly has been translated to the logistics scenario as well.
It was possible to notice, however, that 15 papers did not apply or specify any tools used.This is possibly due to a lack of a clearly defined range of tools.Typically, tools originally applied in a lean or six sigma production context must be adapted in their definitions so they can be correctly applied in the logistics context, addressing its particularities.

RQ-5: What is the productive/industrial/economic sector the article focuses on?
As shown in Figure 9, the most prominent economic sector was the Automotive Industry, which accounted for 24% of all papers, followed by Metallurgical, Equipment and Machinery, Health, Food, Electronics and Air Transportation.13 papers did not clearly define the sector of application.In the "Others" category, with 6 papers, we grouped those less representative sectors.The fact that the automotive industry was the birthplace of lean thinking, justifies the prominence of this sector.Moreover, the high absolute quantity in small gains, due to the high value added in automobiles, as well as the high competitiveness level of this sector, are strong reasons for such result.The results of this RQ state the flexibility of LL and SSL application, offering application opportunities and improvements in several sectors.Therefore, there is opportunity to expand studies that address LL and SSL in different sectors, such as Metallurgical, Health, Food, Equipment and Machinery, among others.

RQ-6: What are the wastes to be eliminated by lean or six sigma logistics addressed by the article?
In Figure 10, the distribution of the 67 selected papers regarding wastes, agreeing with the seven wastes defined by Goldsby et Martichenko (2005), can be observe.On this matter, 47 papers considered the seven wastes approach in its analysis.However, in 20 papers it was not possible to identify which waste the paper was focused on or the waste analysis was not the paper's approach.

CONCLUSION
The analysis of previously published research studies related to Lean and Six Sigma Logistics by means of systematic review denoted the increasing popularity of this topic in several productive and industrial sectors, as for example, in the Automotive industry, being the most frequently approached, as well as the current state of the art regarding theoretical and applied studies.
Despite its popularity in many areas, the relationship between Logistics and Lean Thinking or Six Sigma is still fragile: most papers do not link these concepts in their literature review.
Regarding the LL and SSL applications, we observed that practical applications were published in higher quantity than theoretical studies.The practical papers comprise mainly the implementation and impacts of the LL and SSL philosophy.Hence, this context generates opportunities for more theoretical contributions.This research gap is possibly due to the fragile integration -as mentioned previously -between the concepts that originates LL and SSL: Logistics and Lean Thinking; and Logistics and Six Sigma.
Regarding the tools, Kanban is presented as the most used, which is expected, since it is one of the main Lean tools.On the analysis of wastes, most of the papers referred to Motion and Waiting losses, which shows the relevance of the LL and SSL implementation for waste elimination and reduction.In Table 4, the results of each research question and their respective opportunities of research are described, based on the results analysis.
Through the analysis of the most recent researches published related to LL and SSL, following the systematic review methodology, it was possible to evaluate trends related to: productive and industrial sectors, logistics operational drivers, use of logistics tools, and elimination of wastes that motivate LL and SSL implementation.Moreover, research gaps that may provide opportunities for the development and improvement of research were identified.Therefore, future studies can be developed, following the lines proposed as opportunities, in order to contribute to the LL and SSL field.

RQ-1
Relationship between theoretical concepts of Logistics and Lean Thinking or Six Sigma is still fragile, since they were often approached separately in the papers' literature review.

Research that effectively relate the concepts of Logistics and Lean
Thinking or Six Sigma, enabling the construction of new knowledge about LL and SSL, linking the topics in an integrated way.

RQ-2
Among the logistics operational drivers considered, Transportation was approached in most of papers, both individually or combined with other drivers.
Theoretical and practical studies directed to identify and apply LL and SSL principles and concepts in other logistics drivers (Inventory, Facility and Information), individually or, specially, combined (integrated) with each other, including Transportation.

RQ-3
The majority of LL and SSL publications were applied studies (not theoretical studies), considering the implementation of the Lean or Six Sigma Philosophy.
Theoretical studies on LL and SSL and their applicability in different sectors or contexts, considering possible adaptations and the major error risks already made in order to prevent them.

RQ-4
The main tools used were: Kanban, Milk Run and combination of Six Sigma methods; there was no clear definition in terms of which tools are also LL and SSL tools.
Application of less widely used Lean tools; Studies that define the range of LL and SSL tools, considering both logistics and Lean or Six Sigma tools, with their respective adaptations

RQ-5
The automotive sector was scenario for most LL and SSL applications, demonstrating that other industries still have a certain lack of applications in this sense.
Studies and research applied to other productive and industrial sectors, for example, in healthcare, disseminating Lean Healthcare ideas within a logistic context.

RQ-6
Many publications have not defined or mentioned the wastes considered in the productive systems under study.Moreover, time was the main type of waste identified and eliminated, while waste of packing, knowledge and administration were the less identified ones.
Studies and research that clearly address the wastes of the lean philosophy and six sigma in the logistic context, as well as its importance, providing a dense theoretical basis on such wastes, so that the process of identifying wastes in practical contexts can be simplified.
Source: The author(s)' own

Figure 1 .
Figure 1.Methodology steps for the proposed systematic review Source: Adapted from Negrão et al. (2016)

Figure 2 .
Figure 2. Criteria and coding for each research question Source: The author(s)' own

Figure 3 .
Figure 3. Classification of papers regarding their language Source: The author(s)' own

Figure 4 .
Figure 4. Relationship between logistics and lean concepts Source: The author(s)' own

Figure 5 .
Figure 5. Operational logistics drivers in lean logistics' articles Source: The author(s)' own

Figure 6 .
Figure 6.Quantity of papers per logistics components Source: The author(s)' own

Figure 7 .
Figure 7. Classification of the papers regarding their applicability Source: The author(s)' own

3. 4
RQ-4: What are logistics tools and/or the principles the article addresses?Within the selected papers, 33 different logistics tools/ principles could be identified.The papers often addressed more than one tool at a time in their content.One can observe the distribution of papers regarding their tools in Figure 8, containing the 12 most frequently used ones.

Figure 8 .
Figure 8. Lean and logistics tools broached in the articles Source: The author(s)' own The "Others" category aggregated the less frequently used tools: A3 Problem Solving, Overall Equipment Effectiveness (OEE), Batch size reduction, Teamwork, Total Productive Maintenance (TPM), Radio Frequency Identification (RFID), Variation Control, Spaghetti Diagram, Electronic Data Interchange (EDI), Enterprise Resource Planning (ERP), Continuous flow, Workplace Management, Global Operating Income Index (IROG), Logistics Personal Excellence, Performance Measure System, Plan for Every Part, Single Minute Exchange of Die (SMED), SIPOC, Vendor Managed Inventory (VMI), Water Spider, Constant Work in Progress, 5W2H.

Figure 9 .
Figure 9. Top 5 industrial sectors in which LL concepts are being adopted Source: The author(s)' own

Figure 10 .
Figure 10.Distribution of papers regarding LL waste approach in their content Source: The author(s)' own In Figure 11, it is shown the distribution of those 47 papers in each one of the wastes: Time, Transportation, Inventory, Space and Facilities, Administration, Knowledge and Packing.Most papers addressed more than one waste at a time, i. e., combined different wastes.The waste of time was the most common, in 28 papers, followed by transportation waste, and Inventory waste.The waste caused by packing activities were the less representative, tackled in 3 papers.

Figure 11 .
Figure 11.Papers that reduced or eliminated wastes in their LL and SSL studies Source: The author(s)' own

Table 1 .
Review protocol parameters for the systematic review on LL and SSL RESEARCH PROTOCOL Main Research Question: "

Table 2 .
Research questions and their objectives

Table 3 .
Classification and summary of the lean logistics papers

Table 4 .
Achieved results and research opportunities for each research question